The present invention relates to a process for the preparation of C.sub.7 and/or C.sub.8 aromatic hydrocarbons of high purity. More particularly, the present invention provides a process for the production of toluene, having an aromatic hydrocarbon purity of greater than 95 liquid volume percent, and/or mixed xylenes having an aromatic hydrocarbon purity of greater than 99 liquid volume percent, by treatment of petroleum feed fractions wherein the conventional costly solvent extraction step is completely eliminated, and wherein the yield of these C.sub.7 and/or C.sub.8 aromatic hydrocarbons, per volume of crude petroleum feed, is greatly increased over that obtained with conventional processes.
C.sub.7 and/or C.sub.8 aromatic hydrocarbons of high purity have many essential uses in the chemical industry. It is well known that these hydrocarbons can be formed from the naphthene and/or paraffin hydrocarbons occurring in the naphtha sources, such as from cracking, etc., by catalytic reforming of petroleum fractions under conditions effective to remove hydrogen atoms from the naphthene rings and other reforming type reactions to thereby convert them to aromatic compounds. However, in conventional high severity reforming operations, significant quantities of these nonaromatic compounds are not substantially converted to aromatics. These unconverted nonaromatic compounds boil within the respective C.sub.7 and/or C.sub.8 aromatic hydrocarbon boiling range, and therefore cannot be separated from the aromatic hydrocarbon product by low-cost fractional distillation without also utilizing high-cost solvent extraction. Reforming of naphtha fractions by conventional processes, therefore, produces a C.sub.7 and/or C.sub.8 aromatic hydrocarbon product containing a significant quantity of difficultly removable non-aromatic material. Accordingly, in order to produce a C.sub.7 and/or C.sub.8 aromatic hydrocarbon of commercial quality, it is conventional to subject the resulting reformate to a costly solvent extraction step in order to obtain a high-purity C.sub.7 and/or C.sub.8 aromatic hydrocarbon. Due to the higher cost attendant solvent extraction, including the greater energy requirement therefor, efforts have been made to develop processes for the production of aromatic hydrocarbons which do not require a solvent extraction step in order to produce a product of commercially acceptable quality.
Several processes have been developed for the production of C.sub.7 and/or C.sub.8 aromatic hydrocarbons of commercial purity which dispense with solvent extraction. Typically, this result has been achieved by employing as the reformer charge fraction a hydrocarbon heartcut containing only those aromatic precursors which have a lower boiling point than the aromatics to be produced therefrom, in order to allow the facile separation of the unconverted nonaromatic material and the C.sub.7 and/or C.sub.8 aromatic hydrocarbons. For example, in U.S. Pat. No. 3,635,815, a naphtha feed fraction is prefractionated into an overhead fraction having an upper endpoint of 270.degree. F. to 275.degree. F. (ASTM) and a bottom fraction having a higher endpoint. The overhead fraction is then catalytically reformed under reforming conditions of sufficient severity to convert the lower boiling naphthenes and paraffins to C.sub.8 aromatic which boils above the major part of the heartcut. The resulting reformate is then subjected to a plurality of fractionation steps to produce a mixture of high-purity C.sub.8 aromatic hydrocarbons.
Similarly, in U.S. Pat. No. 3,499,945, a petroleum naphtha fraction is fractionated to produce a C.sub.7 containing heartcut boiling between about 175.degree. and 220.degree. F. The boiling point of this heartcut is significantly less than the 231.degree. F. boiling point of toluene. The C.sub.7 heartcut is reformed to convert toluene precursors, such as the C.sub.7 naphthenes, into toluene, yielding a reformate which is distilled to produce a fraction rich in toluene, but also containing paraffins. High severity thermal cracking, fractionation, and clay treatment of the toluene rich fraction then yields a high-purity toluene product.
U.S. Pat. No. 2,653,175 describes a split-feed reforming process for the preparation of aromatic hydrocarbons in which a petroleum feed is separated into a C.sub.6 and C.sub.7 naphthene heartcut, and a C.sub.8 naphthene heartcut. Each heartcut is separately reformed and separated from similar boiling paraffins by contact with an aromatic selective absorbent.
While the above processes produce C.sub.7 and/or C.sub.8 aromatic hydrocarbons of adequate purity, these processes possess certain disadvantages which render their use undesirable. In each of the above processes, the petroleum feed fraction is prefractionated into very narrow boiling range heartcuts in order to remove the nonaromatic material which boils within the boiling range of the aromatic to be produced from the feed. Prefractionation of the petroleum feed fraction into such very narrow boiling range fractions, however, removes significant quantities of C.sub.7 and/or C.sub.8 aromatic precursors from the conversion process and correspondingly reduces the yield of C.sub.7 and/or C.sub.8 aromatic hydrocarbons per volume of petroleum feed. These prior art processes, therefore, achieve increased purity of the aromatic product at the expense of yield.
It is also known in the art that a two-step reforming process may be employed for the production of aromatic hydrocarbons in which a naphtha feed is reformed under mild conditions in a first step and then subjected to thermal cracking in a second step. Hitherto, however, even with the use of such a reforming procedure in conventional processes, C.sub.7 and/or C.sub.8 aromatic hydrocarbons of less than desirable purity have been obtained. For example, in U.S. Pat. No. 3,499,945, the combination of a prefractionation step and a two-step reforming process fails to achieve a toluene product of commercially acceptable purity without a subsequent clay treatment purification step.
In view of our ever declining supplies of petroleum, the low yields per volume of petroleum feed and/or low aromatic purities obtained with the above processes renders their use undesirable. Accordingly, there exists a great need in the art for a process for the manufacture of high-purity C.sub.7 and/or C.sub.8 aromatic hydrocarbons which eliminates the necessity for costly solvent extraction and which produces a product of commercially acceptable purity with a maximum yield per volume of petroleum feed.